"Why did Hillary get rid of her middle name, huh?" Trump asked during a town hall in Roanoke, Va., repeating "rotten" several more times instead of "Rodham."
"Hillary Rotten Clinton, right? Maybe that's why: It's too close," Trump said.
Trump has for months referred to Clinton as "Crooked Hillary."
Rhetorical attacks on the presumptive Democratic presidential nominee escalated last week, with chants of "lock her up" emerging at the Republican National Convention in Cleveland. Democrats and some Republicans rebuked those calls.
Several news outlets last year announced they were dropping "Rodham" from coverage when referring to Hillary Clinton, then the Democratic presidential front-runner.
The New York Times and The Associated Press said in November that her campaign had confirmed she prefers Hillary Clinton to Hillary Rodham Clinton, as the outlets had previously referred to the former secretary of State.
Clinton has dropped "Rodham" or used some combination of it over the years. When she ran for president in 2008, she ran as Hillary Clinton, but the cover of her 2014 memoir says Hillary Rodham Clinton.